Carbon nanotube has a cylindrical tube structure which is formed by binding neighboring three carbon atoms to one carbon atom, making a hexagonal ring shape, and being rolled up a sheet repeated with the hexagonal ring shape in a beehive form. The carbon nanotube has excellent properties that it is thermally, chemically and mechanically very stable and its electric features vary greatly depending on the structure. Its application fields are very broad including nano electric devices, electric field emission emitter, hydrogen and ion storage, composite, catalytic supporter, sensor and so on.
The currently known methods for preparing the carbon nanotube are as follows. In the early stage of developing methods for producing nanotube, a small amount of the carbon nanotube for studying its structure and electric features was prepared by using arc discharge and laser deposition methods. Since then, several methods including arc discharge, laser evaporation, CVD (chemical vapor deposition), plasma synthesis methods have been developed for the mass production of carbon nanotube (Chemical Physics Letters, Vol. 376, (5-6) 606-611, 2003; Surface and Coatings Technology, Vol. 174-175, (9-10) 81-87, 2003; U.S. Pat. No. 5,424,054; U.S. Pat. No. 6,210,800; U.S. Pat. No. 6,221,330; International Patent Publication No. WO99/006618 A1). The conventional methods have prepared the carbon nanotube in small quantities under a severe reaction condition such as at a high temperature ranging from hundreds and thousands degrees or in a controlled vacuum condition, and required the use of expensive equipments. Therefore, there still exist many problems in the conventional methods for commercially producing the carbon nanotube.
Meanwhile, when a pure liquid phased-material is heated and pressurized to a critical temperature (Tc) and a critical pressure (Pc), its phase has an intermediate and unique feature between liquid and gas phase.
This single-phase is defined as a supercritical fluid, and the fluid's state may be characterized by the critical temperature and critical pressure.
The supercritical fluid has both a gas-like delivery feature and a liquid-like soluble feature, and is capable of approaching to a particular range of physicochemical features such as density, diffusion rate and dielectric constant by appropriately regulating its temperature and pressure without undergoing a change in phase boundary from liquid to gas. Since the supercritical fluid has been regarded as a solvent which can successively regulate its property due to the above mentioned features and shows unique, intermediate characteristics of liquid and gas, it has been identified as a material of interest for a variety of scientific fields.
The present inventors have therefore endeavored to solve the problems of conventional methods for preparing the carbon nanotube from a liquid phased-carbon source such as pyrogenic synthesis, high cost, low productivity and small scale production. As a result, the present inventors have developed a method for preparing carbon nanotube from a liquid phased-carbon source which uses a liquid phased-hydrocarbon based material as carbon source, which exists in a liquid phase at room temperature or under a state of heating and pressurization, and comprises the steps of reacting and cooling the carbon source in the presence of a metal seed catalyst to induce the growth of carbon nanotube crystal under the condition of maintaining a supercritical fluid state by regulating a reaction condition into the range of a critical temperature and a critical pressure of the carbon source used.